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<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></b></td>
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<p align=center><b>Logically renumbering records</b></p>
<p>Records stored in the Queue and Recno access methods are accessed by
logical record number.  In all cases in Btree databases, and optionally
in Recno databases (see the <a href="../../api_c/db_set_flags.html">DB-&gt;set_flags</a> method and the
<a href="../../api_c/db_set_flags.html#DB_RENUMBER">DB_RENUMBER</a> flag for more information), record numbers are
mutable.  This means that the record numbers may change as records are
added to and deleted from the database.  The deletion of record number
4 causes any records numbered 5 and higher to be renumbered downward by
1;  the addition of a new record after record number 4 causes any
records numbered 5 and higher to be renumbered upward by 1.  In all
cases in Queue databases, and by default in Recno databases, record
numbers are not mutable, and the addition or deletion of records to the
database will not cause already-existing record numbers to change.  For
this reason, new records cannot be inserted between already-existing
records in databases with immutable record numbers.</p>
<p>Cursors pointing into a Btree database or a Recno database with mutable
record numbers maintain a reference to a specific record, rather than
a record number, that is, the record they reference does not change as
other records are added or deleted.  For example, if a database contains
three records with the record numbers 1, 2, and 3, and the data items
"A", "B", and "C", respectively, the deletion of record number 2 ("B")
will cause the record "C" to be renumbered downward to record number 2.
A cursor positioned at record number 3 ("C") will be adjusted and
continue to point to "C" after the deletion.  Similarly, a cursor
previously referring to the now deleted record number 2 will be
positioned between the new record numbers 1 and 2, and an insertion
using that cursor will appear between those records.   In this manner
records can be added and deleted to a database without disrupting the
sequential traversal of the database by a cursor.</p>
<p>Only cursors created using a single <a href="../../api_c/db_class.html">DB</a> handle can adjust each
other's position in this way, however.  If multiple <a href="../../api_c/db_class.html">DB</a> handles
have a renumbering Recno database open simultaneously (as when multiple
processes share a single database environment), a record referred to by
one cursor could change underfoot if a cursor created using another
<a href="../../api_c/db_class.html">DB</a> handle inserts or deletes records into the database.  For
this reason, applications using Recno databases with mutable record
numbers will usually make all accesses to the database using a single
<a href="../../api_c/db_class.html">DB</a> handle and cursors created from that handle, or will
otherwise single-thread access to the database, for example, by using
the Berkeley DB Concurrent Data Store product.</p>
<p>In any Queue or Recno databases, creating new records will cause the
creation of multiple records if the record number being created is more
than one greater than the largest record currently in the database. For
example, creating record number 28, when record 25 was previously the
last record in the database, will implicitly create records 26 and 27
as well as 28.  All first, last, next and previous cursor operations
will automatically skip over these implicitly created records.  So, if
record number 5 is the only record the application has created,
implicitly creating records 1 through 4, the <a href="../../api_c/dbc_get.html">DBcursor-&gt;get</a> method with the
<a href="../../api_c/dbc_get.html#DB_FIRST">DB_FIRST</a> flag will return record number 5, not record number 1.
Attempts to explicitly retrieve implicitly created records by their
record number will result in a special error return,
<a href="../../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a>.</p>
<p>In any Berkeley DB database, attempting to retrieve a deleted record, using
a cursor positioned on the record, results in a special error return,
<a href="../../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a>.  In addition, when using Queue databases or Recno
databases with immutable record numbers, attempting to retrieve a deleted
record by its record number will also result in the <a href="../../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a>
return.</p>
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